A cellular or trunked communication system is one in which mobile or portable user terminals, such as mobile telephones or portable or vehicle mounted radios, herein collectively referred to as ‘mobile stations’ or ‘MSs’, can communicate via a system infrastructure which generally includes one or more fixed base stations (base transceiver stations) and other routing and control installations. Each base station has one or more transceivers and serves MSs in a given region or area known as a ‘cell’ by wireless communication. The cells of neighbouring base stations are often overlapping.
A mobile communication system providing wide area coverage may be considered as being formed of a plurality of interlinked networks. Each network normally includes a group of cells often referred to as a ‘zone’. The infrastructure of each network usually comprises, in addition to the base stations which serve the mobile stations in the respective cells of the zone, a router (which may also be referred to as a switch) which routes communications to and from the network and within the network. The router may be associated with, or form part of, a zone controller which may provide other management functions within the network, such as providing management of the base stations on a network level. The infrastructure may also include an authentication processor which authenticates and registers MSs to use the network. The networks of different zones, particularly the routers and authentication processors of those networks, may communicate by various known means such as radio or microwave communication, hard wired electrical or optical communication, or the internet.
It is usual for a MS of a particular user registered with a mobile system operator to have a ‘home’ network which normally provides a communication service to the user. If the user moves to another region not covered by the home network, e.g. to a different part of the user's country or to a foreign country, it is still possible for the user to receive a service from the local network as a visited network. An authentication process involving the user's home network and the visited network usually needs to be completed satisfactorily together with registration of the visiting MS by the visited network.
For example, one particular type of mobile communication system widely used in Europe and elsewhere to support communications within organisations such as public safety services and enterprises is a TETRA system. Such a system is one designed to operate in accordance with the TETRA (Terrestrial Trunked Radio) standard procedures or ‘protocol’ defined by the European Telecommunication Standards Institute (ETSI). Generally, TETRA systems support migration of MSs from a home network to a visited network and provision of communication services in the visited network. Another system which is designed for use in a similar manner is an APCO 25 system which is a system operating according to the APCO 25 standard defined by the Association of Public Safety Communications Officials International (APCO), standardized by the US Telecommunications Industry Association (TIA).
Usually, when a MS requests registration by a visited network, the particular services to be provided to the MS need to be determined by the visited network during the authentication and registration procedure. In known systems, the services to be provided are determined by the visited network obtaining from the home network a service record giving details of the services allowed to be provided to the MS. The services may vary from MS to MS, depending for example on the particular implementation of the MS or on the organisation department or seniority within the organisation or department of the user of the MS. Usually, the visited network provides services which are in line with those allowed in the home network and are specified by the home network when queried by the visited network. Communication between the visited network and the home network of information relating to the allowed services is thus required in known systems. Authentication and registration of the MS by a visited network can be undesirably delayed by the need for such communication, especially where a number of MSs have migrated and requested registration together.
Furthermore, no registration may be possible at all if there is a failure in a communication link between the networks.